Centrifugal pump systems have been implemented for use in a wide variety of applications. Existing centrifugal pump systems typically make use of an overhung impeller. In a typical centrifugal pump system, fluid enters the impeller through the “eye” of the impeller, and is then “centrifuged” to the impeller periphery by the continuous rotation of the impeller, generally with assistance from the impeller vanes. In some designs, impellers can be open or closed, and may have one vane, many vanes, or no vanes at all.
One specific variant of the centrifugal pump system is the liquid-ring pump. Liquid-ring pumps are most commonly used as vacuum pumps or gas compressors. In a liquid-ring pump, liquid (typically water) is fed into the pump, and is formed into a moving cylindrical ring around the inside of the casing by the action of a multi-vane impeller. This liquid ring creates a series of seals in the spaces between the impeller vanes, which form compression chambers. The multi-vane impeller is located slightly off-center from the casing of the liquid-ring pump, such that the eccentricity between the impeller's axis of rotation and the geometric axis of the casing results in a cyclic variation of the volume enclosed by the vanes of the impeller and by the liquid ring. This is used to pump gas (typically air) through the pump; gas is drawn into the pump through an inlet port on one end of the casing, trapped in the compression chambers, reduced in volume by the impeller rotation, and discharged at the other end of the casing.
In current overhung impeller designs for liquid-ring pumps, the radial bearing is located outside of the compressor casing and at some distance from the impeller. Essentially, this means that the most significant portion of the weight of the impeller is cantilevered at some distance away from its point of support. The weight of the impeller being cantilevered this far away from its point of support produces a moment in the shaft, located between the radial bearing and the thrust bearings. This induced moment can be seen in the depiction of a prior art overhung impeller design, shown in exemplary FIG. 1. In current designs, this moment creates a radial load on the thrust bearings, which can lead to premature failure of the thrust bearings.
To further describe the current impeller designs, the impeller is typically attached to the shaft via collar and key, and with an impeller nut. The shaft is typically attached to the center of the impeller. The impeller, as shown, is significantly larger in diameter than the shaft.
During operation, the compressor operates with a certain amount of water inside the casing. When the impeller is active, this water is spun against the side of the casing. However, when the impeller is inactive or idle, this water sits in the bottom of the compressor casing. When the impeller is started from an inactive state, the impeller must turn through the water at the bottom of the compressor casing in order to properly dispose it around the inside of the casing of the compressor. This water creates a significant amount of drag on the impeller.
However, in a startup state, the top of the impeller is surrounded by gas or vapor, which has significantly less viscosity than the water at the bottom of the impeller. This means that there is far less drag on the top of the impeller as compared to the bottom of the impeller. Likewise, the gas at the top of the impeller has far less mass than the water at the bottom of the impeller, and therefore the gas has much less inertia resisting the movement of the impeller than the water. These two factors, combined with the radial bearing to impeller distance, means that the difference in the inertial forces on the impeller that may be required at startup may cause the impeller to “wobble” inside the casing until the impeller reaches a sufficient speed to create the necessary centrifugal motion to distribute the water throughout the casing.
Similar problems may arise when the impeller is to be shut down, though in reverse in this case. As the impeller slows, it will slow past the point at which it can create the necessary centrifugal motion to distribute the water throughout the casing, and the water will increasingly sink to the bottom of the casing. This may cause the bottom portion of the impeller to be slowed more quickly by the inertial force of the water acting on the impeller vanes, again causing wobble in the impeller.
In typical pump designs, the clearance between the impeller and the inside of the casing (specifically, between the impeller and the gas distributor located inside the casing) are very small, often on the order of 0.02″. This means that, when the impeller is started up or shut down and cause to wobble, there is a potential for impact between the impeller and the gas distributor, which can typically damage the distributor, the impeller, or both.
Further, common pump designs as understood in the prior art are often expensive to fabricate and operate. A bearing arrangement for a current liquid ring pump requires that the bearing housing be fabricated and installed on the compressor. It also requires that this bearing be supplemented with an external bearing lubrication method, such as grease, oil, or oil mist, which requires monitoring and periodic refilling, and limits the placement of liquid ring pumps to points where they can be easily accessed for this required maintenance.